Analysing particles in a liquid sample, e.g. determining the concentration and type of the particles, is of great importance in many different areas of industry, such as agriculture, medicine, etc.
The traditional way of studying particles in a liquid sample is visually, possibly with the aid of a microscope if the particles are small. The particle concentration is normally obtained through a manual procedure by microscopically viewing the sample in a special counting chamber, e.g. a Burker chamber. The counting chamber is provided with a grid dividing the chamber in well-defined small volumes. The particles may be allowed to settle at the bottom of the counting chamber in order to enable the microscope to focus on all particles in the chamber and, thus, facilitate counting. Thus, the sample needs to settle for several minutes before the counting may be performed. The particle count can then be determined by counting the number of particles per box in the grid. The particle count is obtained manually by an analyst, who needs to be experienced in performing the analysis in order to be able to perform a reliable analysis.
This analysis is time-consuming. Further, since it is performed manually, the results of the analysis may vary depending on the person performing the analysis. The analysis is also inaccurate since relatively few particles are counted and the volumes of existing counting chambers are often imprecise.
There are a few existing automated analysis methods for determining particle concentrations in liquid samples. Particle concentrations and sizes, especially for biological particles, such as cells, may be determined by means of the Coulter principle, which is based on sensing an impedance. A method for counting white blood cells by the Coulter principle is described in U.S. Pat. No. 5,262,302. A measurement apparatus according to the Coulter principle is expensive and it is therefore a considerable investment. Thus, a hospital or laboratory will be reluctant to invest in more than one apparatus. This implies that the analysis will need to be performed in a centralised location and a patient will need to wait for analysis results.
In WO 98/50777, a method for assessment of the number of somatic cells in milk is disclosed. The method comprises applying a volume of a sample in a sample compartment and transmitting electromagnetic signals, having passed from the sample compartment, onto an array of detection elements. The intensities of detected electromagnetic signals are processed and the results are correlated to the number of cells present in the sample.
The international application WO 2008/010761 discloses an apparatus and a method for enumeration and typing of particles in a sample. The method comprises the steps of acquiring at least one magnified digital image of the sample, identifying particles which are imaged in focus in the image and determining the types and numbers of these particles. When animal cells are the particles, the optical phenomena at the edges of the cells resulting from the cytoplasm and cell membrane acting as a lens are used to identify which cells are imaged in focus. It is also disclosed that images may be acquired at different focus planes in the sample. It is, however, not mentioned how far apart these focus planes should be.
It is still desired to speed up and simplify existing automated methods for analysis of particles in a liquid sample, e.g. a biological sample. It would be particularly advantageous to provide a quick, simple and relatively inexpensive analysis method such that the analysis may be provided at a point of care.